Photoinitiator systems, sometimes known as mixed photoinitiators or co-initiators, have been disclosed for use with polymerizable monomers. It is generally accepted that an initiator compound forms free radicals by interacting with a photosensitizer compound which is capable of absorbing radiation of specific wavelengths. The free radicals formed then initiate polymerization of ethylenically-unsaturated materials that are present in a compositon with the photoinitiator system.
Many oil-soluble photoinitiator systems are known in the art. A well-known oil-soluble photoinitiator system is Michler's ketone, ##STR2## in combination with benzophenone or iodonium salts, a system known to induce high speed photopolymerization. This system cannot be used in water-based coatings due to the insolubility of Michler's ketone in water. U.S. Pat. No. 4,147,604 relates to diaminobenzophenone compounds (none of which have acid or ester functionality) which are useful in photopolymerizable binders and have improved solubility in organic solvents.
Other non-water soluble photoinitiator systems are disclosed in European Patent Application No. 80103303.6 and U.S. Pat. No. 4,147,552, wherein 3-keto-substituted coumarin compounds useful as sensitizers are taught. Also, photosensitizers are disclosed in U.S. Pat. No. 3,728,377 (4,4-bis(alkoxycarbonyl)benzoin ethers) and U.S. Pat. No. 3,988,228 (halogen containing aromatic ketones) and November 1980 RESEARCH DISCLOSURE 19947 (simple bis-quinones).
Very few water-soluble photoinitiator systems which absorb in the UV or near-UV portion of the spectrum (i.e. 320-420 nm) are known. One such system comprising water-soluble derivatives of benzophenone and benzil is disclosed in February 1981 RESEARCH DISCLOSURE 20221. There is need in the art for a high-speed, water-soluble photoinitiator system which is capable of inducing efficient curing of ethylenically-unsaturated compounds for use in water-based lithographic plate technology.